Recent advances in coating technology have provided coatings which are suitable for use over various substrates which are difficult to coat and which have many different properties. Coatings of excellent appearance, a high order of durability and having the ability to withstand severe environmental conditions have been obtained. Among the more advanced coatings are those employed on vehicles, such as automobiles, where good appearance must be maintained over long periods despite exposure to weather and various forms of attack during use.
Recently, there has been interest in the use of rubbery, resilient materials for areas which are subject to mechanical shock, such as automobile bumpers and moldings, exposed corners and surfaces of industrial machines, kickplates and other areas of doors and entrances, and the like. The use of such materials aids in providing protection against permanent structural damage. In order to attain the described appearance, a decorative and protective coating must be applied to the surface and this coating can also be damaged during use. Conventional coatings, including those employed on rubber and similar extensible objects heretofore, do not have the required combination of properties. These necessary properties include:
1. Extensibility--This property is necessary in order that the advantages of the resilient substrates can be utilized without destruction of the integrity of the surface of the coating.
2. Tensile Strength--A high degree of tensile strength is also necessary in order to avoid rupture of the film during use.
3. Package Stability--In order to permit ease of application, the liquid coating composition should be stable for extended periods under ambient conditions without either gelation or depolymerization of the resin contained therein.
4. Film Stability--Certain coatings which are extensible and which have a relatively high tensile strength lose those properties upon aging and particularly if the coating is exposed to sunlight, weathering, etc.
5. Impact Resistance--The coating should have adequate impact resistance at various temperatures as encountered in extreme weather variations, including temperatures as low as -20.degree. F. and as high as 120.degree. F.
6. Adhesion--The coating should have satisfactory adhesion to the various substrates with which it is to be employed, including extensible materials such as foams, rubber and the like, and metals such as mild steel. In addition, the coatings should have satisfactory intercoat adhesion with suceeding coats or with various primers which can be employed.
7. Chemical and Humidity Resistance--This includes properties such as saponification resistance upon exposure to acids and alkalis, resistance to various solvents and resistance to atmospheres of high humidity and heat.
8. Resistance to Cracking Under Temperature-Humidity Cycling--This property is important where the coating might be exposed to rapid variations in temperature and humidity as might be encountered by automobiles during travel or storage. This property is tested by successively exposing the coated object to conditions of high temperature and high humidity alternated with exposure to low temperature and low humidity.
Still other properties which are important for commercial applicability include non-toxicity and low sensitivity to moisture. Finally, the compositions must possess sprayability at reasonable solids content (i.e., 5 percent). Sprayability, as is recognized in the art, is a measure of the minimum amount of solvent necessary to atomize a polymer system sufficiently to produce a uniform coating film. The measure itself is usually expressed (and is expressed herein) as a percent solids. Thus, a sprayability of 5 percent solids would denote a system requiring 95 parts of solvent per 5 parts of solid to atomize. If the solvent concentration is below the minimum, the polymer will form a stringy, web spray pattern. The sprayability of a resin will generally be a close indication of the sprayability of the paint or coating formulation.
It is especially difficult to obtain the above properties in combination since, in most instances, the obtaining of one or several of the properties desired requires the use of materials and formulations which under ordinary circumstances tend to make the other desired properties less satisfactory.